HOLLOW BAR SOIL NAILS - Pullout Test Program Publication No. FHWA-CFL/TD-10-001 May 2010
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HOLLOW BAR SOIL NAILS
Pullout Test Program
Publication No. FHWA-CFL/TD-10-001 May 2010
Central Federal Lands Highway Division
12300 West Dakota Avenue
Lakewood, CO 80228
Technical Report Documentation Page
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
FHWA-CFL/TD-10-001
4. Title and Subtitle 5. Report Date
May 2010
Hollow Bar Soil Nails
Pullout Test Program 6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.
Allen W. Cadden, P.E., Jesús E. Gómez, Ph.D., P.E.,
Andrew C. Baxter, P.G., Thomas Bird
9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)
Schnabel Engineering LLC ADSC
510 East Gay Street 8445 Freeport Parkway, Suite 325 11. Contract or Grant No.
West Chester, PA 19380 Irving, TX 75063 DTFH68-07-P-00092
12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered
Federal Highway Administration Final Report
Central Federal Lands Highway Division July 2007 – May 2010
12300 W. Dakota Avenue, Suite 210 14. Sponsoring Agency Code
Lakewood, CO 80228 HFTS-16.4
15. Supplementary Notes
COTR: Justin Henwood, FHWA-CFLHD. Advisory Panel Members: Matthew Demarco and Roger Surdahl,
FHWA-CFLHD; Scott Anderson and Barry Siel, FHWA-RC; and Rich Barrows, FHWA-WFLHD.
This project was funded by the FHWA Federal Lands Highway Geotechnical Program, the FHWA Federal
Lands Highway Technology Deployment Program, and the Association of Drilled Shaft Contractors – The
International Association of Foundation Drilling.
16. Abstract
The use of Hollow Bar Soil Nails (HBSNs) is growing in the excavation support and retaining wall
construction. It is anticipated that the use of the HBSN technology could reduce construction schedules,
costs, and environmental impacts. The current state of practice for design bond strengths and load testing
procedures is based on the current soil nail practice but varies depending on the installation contractor and
product recommendations. The objective of this study was two-fold. The first objective was to develop an
initial data file from installation and testing at four sites of the available grout-to-ground bond stress of
HBSNs, and to determine if correlations exist with traditional solid bar, drill, and grout soil nails (for
example, the published nominal values in FHWA-IF-03-017 [GEC No. 7]). The second objective was to
establish recommendations for practical, standard ways of performing pullout tests on HBSNs.
Comparisons between the pullout test results showed that the HBSNs generally developed larger bond
strength values in granular soils than the SBSNs. Three installation methods for purposes of pullout and
proof testing were evaluated. Of which two were found to be to be satisfactory; however, the third one noted
to have a significant Doughnut Effect and was not recommended for pullout testing.
Test results of the four sites are included in detail on the attached CD ROM.
17. Key Words 18. Distribution Statement
HOLLOW BAR SOIL NAILS, LOAD No restriction. This document is available to the
TESTING, SOIL NAILS, HOLLOW BAR public from the sponsoring agency at the website
MICROPILES http://www.cflhd.gov.
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price
Unclassified Unclassified 427
(60 printed)
(367 on CD ROM)
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
SI* (MODERN METRIC) CONVERSION FACTORS
APPROXIMATE CONVERSIONS TO SI UNITS
Symbol When You Know Multiply By To Find Symbol
LENGTH
in inches 25.4 millimeters mm
ft feet 0.305 meters m
yd yards 0.914 meters m
mi miles 1.61 kilometers km
AREA
in2 square inches 645.2 square millimeters mm2
ft2 square feet 0.093 square meters m2
yd2 square yard 0.836 square meters m2
ac acres 0.405 hectares ha
mi2 square miles 2.59 square kilometers km2
VOLUME
fl oz fluid ounces 29.57 milliliters mL
gal gallons 3.785 liters L
ft3 cubic feet 0.028 cubic meters m3
yd3 cubic yards 0.765 cubic meters m3
NOTE: volumes greater than 1000 L shall be shown in m3
MASS
oz ounces 28.35 grams g
lb pounds 0.454 kilograms kg
T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t")
TEMPERATURE (exact degrees)
°F Fahrenheit 5 (F-32)/9 Celsius °C
or (F-32)/1.8
ILLUMINATION
fc foot-candles 10.76 lux lx
fl foot-Lamberts 3.426 candela/m2 cd/m2
FORCE and PRESSURE or STRESS
lbf poundforce 4.45 newtons N
lbf/in2 poundforce per square inch 6.89 kilopascals kPa
APPROXIMATE CONVERSIONS FROM SI UNITS
Symbol When You Know Multiply By To Find Symbol
LENGTH
mm millimeters 0.039 inches in
m meters 3.28 feet ft
m meters 1.09 yards yd
km kilometers 0.621 miles mi
AREA
mm2 square millimeters 0.0016 square inches in2
m2 square meters 10.764 square feet ft2
m2 square meters 1.195 square yards yd2
ha hectares 2.47 acres ac
km2 square kilometers 0.386 square miles mi2
VOLUME
mL milliliters 0.034 fluid ounces fl oz
L liters 0.264 gallons gal
m3 cubic meters 35.314 cubic feet ft3
m3 cubic meters 1.307 cubic yards yd3
MASS
g grams 0.035 ounces oz
kg kilograms 2.202 pounds lb
Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T
TEMPERATURE (exact degrees)
°C Celsius 1.8C+32 Fahrenheit °F
ILLUMINATION
lx lux 0.0929 foot-candles fc
cd/m2 candela/m2 0.2919 foot-Lamberts fl
FORCE and PRESSURE or STRESS
N newtons 0.225 poundforce lbf
kPa kilopascals 0.145 poundforce per square inch lbf/in2
ii
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
______________________________________________________________________
TABLE OF CONTENTS
CHAPTER 1 – INTRODUCTION ..................................................................................................1
CHAPTER 2 – BACKGROUND ....................................................................................................3
FACTORS AFFECTING BOND STRENGTH OF SOIL NAILS......................................3
HBSN BOND STRENGTH AND ITS MEASUREMENT ................................................5
CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM ...................................................7
INSTALLATION, TESTING, AND ANALYSIS PROGRAM .........................................7
SITE SELECTION ..............................................................................................................7
SITE DESCRIPTIONS........................................................................................................8
Site 1: Block 76 Site (Details are shown in the Appendix 1
on the attached CD ROM) ..................................................................................8
Site 2: Posillico Site (Details are shown in the Appendix 2
on the attached CD ROM) ..................................................................................8
Site 3: Sunset Mesa Site (Details are shown in the Appendix 3
on the attached CD ROM) ..................................................................................8
Site 4: Olympia Site (Details are shown in the Appendix 4
on the attached CD ROM) ..................................................................................9
SOIL NAIL INSTALLATION METHOD..........................................................................9
Method A: Traditional Test SBSN Installed Using Casing ..................................9
Method B: Hollow Bar Installed with Debonding Sheath
and Water Flushing ...........................................................................................11
Method C: Hollow Bar Installed with Debonding Sheath
and Without Flushing........................................................................................13
Method D: Hollow Bar with Bondbreaker Installed by Re-drilling a
Pre-grouted Hole...............................................................................................14
CONSTRUCTION DETAILS: MATERIALS, INSTRUMENTATION, AND
INSTALLATION ..............................................................................................................15
Materials ................................................................................................................15
Strain Gauges 17
Equipment and Installation ....................................................................................18
SOIL NAIL TESTING ......................................................................................................20
Load Testing Setup ................................................................................................21
Strain Gauge Use at Olympia Site .........................................................................21
CHAPTER 4 – FILED TEST RESULTS ......................................................................................23
OVERVIEW OF DATA ....................................................................................................23
Load Test Interpretation Methods..........................................................................28
iii
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
______________________________________________________________________
CHAPTER 5 – ANALYSIS AND INTERPRETATION..............................................................29
ANALYSIS OF INTERPRETED BOND STRENGTH DATA .......................................29
Exhumed Test Nails...............................................................................................34
Discussion on Interpreted Bond Values.................................................................35
The Doughnut Effect..............................................................................................36
Strain Gauge Data ..................................................................................................38
CHAPTER 6 – CONCLUSIONS ..................................................................................................43
REFERENCES ..............................................................................................................................47
APPENDIX 1 – BLOCK 76 SITE DATA.....................................................................................49
APPENDIX 2 – POSILLICO SITE DATA.................................................................................135
APPENDIX 3 – SUNSET MESA SITE DATA ..........................................................................229
APPENDIX 4 – OLYMPIA SITE DATA ...................................................................................331
(Appendices are located on the attached CD ROM)
iv
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
______________________________________________________________________
LIST OF FIGURES
Figure 1. Photograph. Installation (Method A) of a solid bar at the Posillico Site. .............10
Figure 2. Schematic. Solid bar soil nail installed using Method A. .....................................10
Figure 3. Photograph. Outlet end of flushing tube. ..............................................................11
Figure 4. Schematic. HBSN installed using Method B........................................................12
Figure 5. Photograph. Grout remaining in the casing after flushing at the
Sunset Mesa Site. ..............................................................................................12
Figure 6. Schematic. HBSN installed using Method C........................................................14
Figure 7. Schematic. HBSN installed using Method D........................................................15
Figure 8. Photograph. Obermann high shear mixer grout pump..........................................17
Figure 9. Photograph. Geokon 4200-AX strain gauge with threadbar attachment
and notched dumbbells. ....................................................................................18
Figure 10. Photograph. Casing installation at the Sunset Mesa Site......................................19
Figure 11. Photograph. Top hammer setup used for installation Methods B and C.
Note the grout pressure gauge at the swivel and centralizer used to hold
PVC in place. ....................................................................................................19
Figure 12. Photograph. Load test setup at Sunset Mesa Site. ................................................21
Figure 13. Graph. Interpreted bond strength for the HBSN and SBSN. ................................30
Figure 14. Graph. Normalized interpreted bond strength. .....................................................32
Figure 15. Photograph. Exhumed HBSN at the Sunset Mesa Site.........................................35
Figure 16. Schematic. Geometry and resistance mechanism of the Doughnut Effect. ..........37
Figure 17. Graph. Load distribution from strain gauges in HBSN B1 (Olympia Site)..........38
Figure 18. Graph. Load distribution from strain gauges from HBSN C2 (Olympia Site). ....39
Figure 19. Graph. Load distribution from strain gauges from HBSN D2 (Olympia Site).....39
Figure 20. Graph. Measured load within the free length of the soil nail from the
Olympia Site .....................................................................................................40
v
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
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LIST OF TABLES
Table 1. Estimated bond strength of soil nails in soil and rock .............................................4
Table 2. Soil nail material summary ....................................................................................16
Table 3. Typical verification test load schedule ..................................................................20
Table 4. Block 76 load test data summary ...........................................................................24
Table 5. Posillico load test data summary............................................................................25
Table 6. Sunset Mesa load test data summary .....................................................................26
Table 7. Olympia load test data summary............................................................................27
Table 8. Range of bond strength by soil type and installation method................................33
Table 9. Bond strength comparison .....................................................................................34
Table 10. Range of bond zone diameters measured from exhumed nails..............................35
Table 11. Comparison of strain gauge data to deformation data ...........................................41
vi
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
______________________________________________________________________
LIST OF ABBREVIATIONS AND SYMBOLS
α Bond strength = Ultimate shear strength of grout-to-ground interface
A Cross sectional area of the steel reinforcing bar
AASHTO American Association of State Highway and Transportation Officials
ADSC Association of Drilled Shaft Contractors – The International Association of
Foundation Drilling
AER Anchored Earth Retention
AL Alignment Load
ASTM American Society for Testing and Materials
Dnom Nominal diameter of the drill hole [in]
DTL Design Test Load [kip]
E Young’s Modulus of steel, 29,000 ksi
FHWA Federal Highway Administration
ft Foot (feet)
Fy Yield (nominal) strength of hollow bar soil nail
GEC Geotechnical Engineering Circular
HBSN Hollow Bar Soil Nail
ID Inside diameter
in Inches
kPa Kilo Pascal
Kip 1000 lb
LBV Test nail bonded length [ft]
MTL Maximum Test Load
OD Outside diameter
PVC Poly Vinyl Chloride
Qultimate Pullout load [lb]. If pullout does not occur at the maximum test load, it is noted
with (*) in the table
SBSN Solid Bar Soil Nail
SOP State of Practice
~ Approximately equal to
vii
HOLLOW BAR SOIL NAILS PULLOUT TEST PROGRAM – TABLE OF CONTENTS
______________________________________________________________________
ACKNOWLEDGMENTS
The authors of this report would like to extend a debt of gratitude to many people and
organizations that provided valuable input into this effort. First, The Association of Drilled Shaft
Contractors – International Association of Foundation Drilling (ADSC) anchored this effort
through their Anchored Earth Retention Committee lead by Kyle Camper and Tom Bird.
Numerous participants from Schnabel Engineering LLC, involved in this effort included Javier
Rodríguez, P.E., Johanna Mikita, Mark Rohrbach, and Helen Foreman.
This work also could not have been completed without the in-kind contributions from
numerous contractors including: Nicholson Construction Company - Western District, Williams
Form Engineering Corp., Con-Tech Systems Ltd., DSI America, DBM Contractors Inc. - Pacific
Northwest Region, TEI Rock Drills Inc., Mountain Highwall Concrete Contractors Inc., Peterson
Geotechnical Construction LLC, and Posillico Construction Inc.
Special thanks are also extended to the review team for this effort:
Mountain Highwall Concrete Contractors Inc.
TEI Rock Drills Inc.
Posillico Construction Inc.
Peterson Geotechnical Construction LLC
HBSN Study Review Team:
Bernard Froemel – DSI America
Lucian Bogdan – DSI America
Cary Lange – Nicholson Construction Company
Dan Thome – Nicholson Construction Company
Tom Richards – Nicholson Construction Company
Donald Bruce – Geosystems L.P.
Horst Aschenbroich – Con-Tech Systems Ltd.
David Weatherby – Schnabel Foundation Co.
Jon Bennett – Geostructures Inc.
Joe Patterson – TEI Rock Drills Inc.
Tom Printz – Williams Form Engineering Corp.
Michael Moore – ADSC
Jerold Bishop – Geotechnical Design Services Inc.
John R. Wolosick – Hayward Baker Inc.
Kyle Vanderberg – Mays Construction Specialties Inc.
Mick Muller – Yenter Companies Inc.
Naresh Samtani – NCS Consultants LLC
Kathryn Griswell – CALTRANS
Robert Carnevale – DBM Contractors Inc.
FHWA Advisory Panel:
Justin Henwood, Matthew Demarco, Roger Surdahl, Scott Anderson, Barry Siel, and
Rich Barrows.
viiiCHAPTER 1 – INTRODUCTION
______________________________________________________________________
CHAPTER 1 – INTRODUCTION
Hollow Bar Soil Nails (HBSNs) differ from Solid Bar Soil Nails (SBSNs) in that HBSNs are
typically advanced through their full design length with a sacrificial bit using grout as the drilling
fluid; thus drilling, grouting, and tendon installation are all accomplished as part of one single
process. Conversely, depending on the soil conditions, SBSNs are installed by pre-drilling a
hole with or without temporary casing, which is then grouted. For SBSNs only, the steel is
introduced before or after grouting depending on the specifics of the construction site. The
Hollow-Core Soil Nails State-of-Practice Report issued by the Federal Highway Administration
(FHWA, 2006) (SOP) provides several areas for further investigation. This report focuses on
additional research and guidance for design of grout-to-ground bond and testing of HBSNs.
HBSNs are typically used in collapsing ground conditions where the simultaneous injection of
grout through the reinforcing bar during drilling keeps the drill hole from collapsing. Due to their
installation method, the procedures for typical solid bar installation and testing are not
appropriate.
This work as discussed hereinafter had two objectives. The first objective was to develop an
initial data file of the available grout-to-ground bond strength of HBSNs, and to determine if
correlations exist with traditional drilled and grouted SBSNs. For this objective, several
practical installation procedures for HBSNs were used to install nails for testing. A total of 39
test HBSNs were installed in four different project sites across the United States. The test data
collected was used to develop the initial database on bond values and to establish
recommendations for the practical installation of test HBSNs.
The second objective was to establish recommendations for practical, standardized methods of
performing comparable pullout tests on HBSNs. These recommendations may include unique
test HBSN design and related standardization installation procedures. This report includes
recommendations for installation, testing, and documentation for HBSNs as a result of this
research work, and recommendations for further study.
1CHAPTER 2 – BACKGROUND
______________________________________________________________________
CHAPTER 2 – BACKGROUND
FACTORS AFFECTING BOND STRENGTH OF SOIL NAILS
HBSNs have been used in practice for over 15 years for both temporary and long-term
excavation support. A description of the application, materials, installation methods, and design
for HBSNs is provided in the Hollow-Core Soil Nails State-of-Practice (SOP) Report issued by
the Federal Highway Administration (FHWA, 2006). In addition, the general design methods for
local failure mechanisms and global stability for soil nail walls are provided in the Geotechnical
Engineering Circular No. 7 (GEC 7), Soil Nail Walls (FHWA, 2003).
The bond strength at a given point along a soil nail corresponds to the shear strength along the
interface between the grout and the ground at that same point. Typically, the interface strength
between flat structural fascia and the ground depends on the following factors (Gómez et al.,
2000, 2003, and 2008):
Effective and total strength parameters of the soil or rock.
Roughness of the grout-ground interface.
Confining stresses.
Rate of loading and soil permeability.
In the case of soil nails, other factors that may influence the average bond strength along a soil
nail include:
Stiffness of the soil or rock.
Diameter of the soil nail, including permeated zone.
Length and axial stiffness of the nail transferring the nail load to the ground.
As the soil nail tends to slide past the soil, the roughness of the interface between the soil nail
and the ground may cause radial expansion of the soil at the nail interface. The normal stresses
acting on the interface may increase, thus increasing the available bond strength under drained
conditions. The magnitude of the normal stress would depend on the bond zone roughness and
on the stiffness of the soil or rock around the nail. The average bond strength also depends on the
interface shear stress level reached at different points along the bond zone. Shorter and stiffer
soil nails will likely favor simultaneous mobilization of bond strength along their entire length;
whereas longer, more flexible nails would favor mobilization of shear strength along their length
at different times and present somewhat lower average bond strength in identical formations.
The current document governing soil nail design and construction, GEC 7, includes a table of
estimated bond strength values for gravity grouted SBSNs installed in soil and rock. The table
was originally compiled by Elias and Juran in 1991 (FHWA, 2003), and is reproduced here as
Table 1. The influence of the ground characteristics and drilling method on the estimated bond
strength is evident as shown in Table 1. Although this table does not reflect all the factors that
affect the bond strength of soil nails, it does provide a useful range of bond values for
preliminary design and is of frequent use in practical design applications. From the authors'
3CHAPTER 2 – BACKGROUND
______________________________________________________________________
experience and as noted in the SOP (FHWA, 2006), HBSNs are initially classified as rotary
drilled when referring to Table 1. As an alternative, jet grouted SBSNs may be similar to
HBSNs; however, the relatively low pressures used in the installation of HBSNs may not result
in an increase of the hole diameter as significant as that typically seen in high pressure jet
grouted SBSNs (FHWA, 2006). The differences in installation and bond strength of SBSNs and
HBSNs will be presented and discussed in a subsequent section of this report.
Table 1. Estimated bond strength of soil nails in soil and rock.
Ultimate Bond
Construction
Material Soil/Rock Type Strength, qu
Method
(kPa)
Marl/limestone 300 - 400
Phyllite 100 - 300
Chalk 500 - 600
Soft dolomite 400 - 600
Fissured dolomite 600 - 1000
Rock Rotary Drilled
Weathered sandstone 200 - 300
Weathered shale 100 - 150
Weathered schist 100 - 175
Basalt 500 - 600
Slate/Hard shale 300 - 400
Sand/gravel 100 - 180
Silty sand 100 - 150
Rotary Drilled Silt 60 - 75
Piedmont residual 40 - 120
Fine colluvium 75 - 150
Sand/gravel
low overburden 190 - 240
Cohesionless
Driven Casing high overburden 280 - 430
Soils
Dense Moraine 380 - 480
Colluvium 100 - 180
Silty sand fill 20 - 40
Augered Silty fine sand 55 - 90
Silty clayey sand 60 - 140
Sand 380
Jet Grouted
Sand/gravel 700
Rotary Drilled Silty clay 35 - 50
Driven Casing Clayey silt 90 - 140
Loess 25 - 75
Fine-Grained
Soft clay 20 - 30
Soils
Augered Stiff clay 40 - 60
Stiff clayey silt 40 - 100
Calcareous sandy clay 90 - 140
From Elias and Juran, 1991 and reproduced in Geotechnical Engineering Circular No. 7 (GEC 7),
Soil Nail Walls (FHWA, 2003).
Notes: Convert values in kPa to psf by multiplying by 20.9
Convert values in kPa to psi by multiplying by 0.145
4CHAPTER 2 – BACKGROUND
______________________________________________________________________
HBSN BOND STRENGTH AND ITS MEASUREMENT
In response to the first objective of this research, we reviewed the factors that affect the bond
strength of the grout to soil or rock as it relates to HBSNs. The higher grout injection pressures
used for installation of HBSNs may lead to either undercutting and permeation of grout in
granular soils or to undercutting of fine-grained soils. It is widely believed that results of higher
grout injection pressures have a positive impact on the grout-to-ground bond strength. The
increase in bond strength will depend on the soil characteristics and on the installation process.
Some of the installation features affecting bond strength include grout mix properties at time of
drilling, injection pressure of the grout, drill bit size and type, and drilling feed rate. Another
important variable would likely be whether a leaner grout (higher water to cement ratio) is used
for drilling, as this may promote more intense permeation of grout in granular soils.
One suggested technique to summarize the factors that promote increased bond strength in
HBSNs during design is to multiply the drill bit diameter by a summary parameter or factor to
account for undercutting and grout permeation (Con-Tech 2009). This factor is dependent on the
soil type being penetrated with the HBSN. The results of the tests performed for this
investigation provide some insight into the suitability of using factored diameter values for the
design of HBSNs.
Extrapolation of bond strength values for typical SBSNs for use in the design of HBSNs can be
difficult. Current soil nail testing includes verification tests and proof tests as part of the testing
program. During installation of a solid bar test nail, the pre-drilled hole is filled with grout only
to a predetermined bond length, leaving an upper unbonded length free of grout. The position
and length of the bond zone in a test nail are selected to test the bond strength in a specific
stratum. Results of the tests enable back calculation of the in situ bond strength of the grout-
ground interface and verification of the design assumptions critical to internal and global
stability of a soil nail wall. For traditional SBSNs, the installation and testing procedures are as
defined in current FHWA documents (for example, FHWA, 2003).
Conversely, the concurrent injection of grout during the drilling of an HBSN results in a
complete column of grout. There are no universally established and accepted installation
procedures to allow consistent creation of bonded and unbonded zones within a test HBSN for
selective bond strength verification. Based on the authors’ experience and as reported in the SOP
(FHWA 2006), the unbonded zone of test HBSNs has been developed by flushing grout, or
through isolation by using a pre-installed smooth casing bondbreaker, or both. Each contractor
develops a different procedure depending on the peculiarities of each job site, the required
unbonded length, the available equipment and materials, and their own preferences. Thus,
reported values of bond strength from differing contractors can vary greatly. Therefore,
development of a suitable installation and testing protocol for HBSNs is a significant need at this
time.
5CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
______________________________________________________________________
CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
INSTALLATION, TESTING, AND ANALYSIS PROGRAM
The FHWA approached the International Association of Foundation Drilling (ADSC) Anchored
Earth Retention (AER) Committee to coordinate an effort to look at the feasibility of using
HBSNs on permanent soil nail wall construction. The AER Committee selected Schnabel
Engineering (Schnabel) as the Principal Investigator for the activity. Using preliminary
installation and test protocols developed by an ADSC AER Task Force and FHWA personnel,
Schnabel finalized a work plan consisting of site selection, soil nail installation techniques, and
testing procedures. Working with the ADSC AER Committee and with the aid of ADSC-
experienced contractors around the United States, Schnabel identified the following four sites,
which were suitable for installation and testing of HBSNs for this investigation:
Site 1: City Creek Center, Block 76 (Block 76 Site), located in Salt Lake City, Utah
Site 2: Posillico Storage Yard (Posillico Site), located in Bohemia, New York
Site 3: Sunset Mesa Gravel Pit (Sunset Mesa Site), located in Montrose County,
Colorado
Site 4: DIS Wheeler Property (Olympia Site), located in Olympia, Washington
At each site, eight to twelve test soil nails were installed and tested in accordance with the
installation, testing, and recording plans established in the program installation protocol (work
plan) dated 1/10/2008. The installation, testing, and recording plans are reiterated in this chapter.
A comparison was made between the calculated bond strength values from each installation
method. In addition, the bond strength values of the test nails were compared to typical bond
values from SBSNs using rotary drilled and jet grouted installation methods. The following
pages contain pertinent details of each of the test sites, and detailed descriptions of the
installation procedures used.
SITE SELECTION
The research team focused on sites where the use of HBSNs would provide practical advantages
with respect to conventionally installed SBSNs (for example, sites where readily loosened
granular soil deposits were present, which necessitated the use of temporary casing during the
drilling and installation of SBSNs). Preferable soil conditions for testing included:
a) Colluvium sites.
b) Sands/Silty sands that are marginally stable for cuts.
c) Clean sands such as coastal beach areas.
It should be noted that HBSNs may be advantageous in other soil deposits not included in this
list.
The four sites selected met the above criteria while representing a variety of geographical and
geological conditions across the United States. In general, the sites ranged from clayey fine sand
7CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
______________________________________________________________________
to poorly graded gravel containing cobbles and boulders. The following sections provide a brief
description of the sites. More detailed information is included in the appended data reports
(Appendices 1-4) that were generated after each test program was completed.
It is noted that the applicability of hollow bars is not limited to granular soils. There are possibly
a number of other geotechnical conditions where HBSNs would be advantageous over SBSNs, or
where SBSNs might require casing for their installation. However, this research focused on sites
where HBSNs would prevent the need for casing for the installation of production nails, and
limit both the potential for collapse of the hole and development of grout body constrictions.
SITE DESCRIPTIONS
The location and soil deposits for each of the four test sites are described in the following
paragraphs. The soil descriptions are based on the soil boring and laboratory test data available
for each site, included in Appendices 1-4, and on-site observation noted on installation logs.
Site 1: Block 76 Site (Details are shown in the Appendix 1 on the attached CD ROM)
Test Site 1 was an active construction site known as City Creek Center – Block 76, and is located
in Salt Lake City, Utah. The site is at the corner of Maine and ES Temple Streets. A temporary
soil nail retaining wall was required for a site access ramp. Nine test nails were installed by
Nicholson Construction near the base of this ramp and were exhumed during final construction.
The exhumed nails were not logged; however, photographs of the nails are included in the
Appendix 1. Based on the observations noted during the installation of the soil nails and
available geotechnical borings, the subsurface conditions consisted of moist to wet, poorly to
well graded gravel of medium to hard density, with silt, sand, cobbles, and the occasional clay
seams. The geotechnical borings are included in the Appendix 1.
Site 2: Posillico Site (Details are shown in the Appendix 2 on the attached CD ROM)
Posillico Construction operates a storage yard in Bohemia, New York, which was used as Test
Site 2 for this research study. Based on the geotechnical boring by Schnabel (Schnabel, 2006),
this site is underlain by relatively clean, fine to coarse sand with layers containing cobbles, as
well as lenses of thin silt or clay. A trench was excavated at this site to allow installation of nails
into the trench wall. Peterson Geotechnical and Posillico Construction installed the nails and
performed the testing. All nails were exhumed after testing.
Site 3: Sunset Mesa Site (Details are shown in the Appendix 3 on the attached CD ROM)
Test Site 3 was located at the active Sunset Mesa Gravel Pit in Montrose County, Colorado. The
gravel pit is used for borrow material on local projects. Based on the geotechnical report by
Buckhorn Geotech, Inc. (Buckhorn, 2008) developed for the test area, the geologic stratigraphy
consists of uniform poorly graded gravel with sand. Buckhorn Geotech, Inc. describes the soil in
which the HBSNs were installed as damp, dense, sandy gravel, cobbles, and boulders with trace
fines.
8CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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A working bench and excavated face was created by Mountain Highwall Construction and eight
nails were installed by Mountain Highwall and TEI Rock Drill for purposes of this testing
program. All nails were exhumed after testing; photographs were provided by Mountain
Highwall for review.
Site 4: Olympia Site (Details are shown in the Appendix 4 on the attached CD ROM)
Test Site 4 was located in Olympia, Washington, and is known as the DIS Wheeler property at
the southeast corner of 14th Avenue and Jefferson Street. The test area was located within the
limits of a temporary stormwater retention area constructed on the north side of the site. This
area was excavated to a depth of about 10 ft with sloping sides. The test area setup was located
along the eastern side of this retention basin and all nails were drilled from the bottom of the
basin. As such, the nails entered the slope about two to three feet above the bottom of the basin.
Based on the available geotechnical site investigation report (Haley & Aldrich, 2008), the
geologic stratigraphy consists of fill overlying recessional outwash/glacio-lacustrine deposits.
The encountered fill material consisted of soft to medium stiff silt and clay with variable debris
and organic matter content. The soil nails were installed just below the fill layer in the
recessional outwash/glacio-lacustrine deposits, which consist of stratified layers of sand and silt
of soft to medium density. Field observation indicated the material into which the soil nails were
installed was generally clayey fine sand.
SOIL NAIL INSTALLATION METHODS
Three methods were originally identified and selected for this test program. Method A is a
baseline for comparison with the conventional SBSN. Methods B and C were conceived as
practical ways to create an unbonded length along a test HBSN. They have been used in some
form or another by contractors in past construction projects. In particular, Method C was
expected to be a readily constructible production testing protocol. Method D was conceived
prior to installation and testing at the fourth site and may be a potential alternate to Methods B
and C. Method D offers certain advantages in some cases as described subsequently in this
report.
Method A: Traditional Test SBSN Installed Using Casing
This is the typical procedure for installation of test SBSNs, where the free length of the soil nail
is achieved with the aid of a temporary casing as shown in Figure 1. For this project, the test
SBSNs were drilled using casing and a drag bit or roller bit. The method of casing advancement
and bit type are indicated on the installation logs, included in the site reports in Appendices 1-4.
After completion of drilling, the rods and drill bit were withdrawn and the tendon with
centralizers was inserted. The tendon was a bare all-thread No. 14 bar (Fy=75 ksi) fitted with a
PVC sheath along the intended free (or unbonded) length, as shown in Figure 2. The intended
bond length below the bottom of the PVC sheath was tremie grouted while controlling grout
volume based on a nominal hole diameter. The casing was then pulled to the top of the bond
zone and the grout level was checked; grout was added or flushed as necessary. Upon
9CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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completion of the soil nail, the temporary casing was withdrawn from the hole, allowing the in-
situ soil to collapse around the nail and smooth PVC sheath. For the test nails installed following
this procedure, there is no grout intentionally surrounding the free length of the SBSN.
Figure 1. Photograph. Installation
(Method A) of a solid bar at the
Posillico Site.
Temporary Casing Smooth
(Withdrawn) PVC Sheath
Possible Raveled Soil
After Grout Flushing
Solid Thread Bar
Structural Grout
Figure 2. Schematic. Solid bar soil nail installed using Method A.
10CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Method B: Hollow Bar Installed with Debonding Sheath and Water Flushing
For Method B, HBSNs were installed by drilling with a temporary casing and a drag bit or roller
bit completely through the unbonded length and stopping at the top of the desired bond zone.
The method of casing advancement and bit type are indicated on the installation logs. After the
casing was installed and the drill bit withdrawn, the hollow bar with a sacrificial drill bit was
inserted into the casing. Drilling continued following typical installation procedures for HBSNs
(as discussed in FHWA, 2006).
To ensure the required free length was truly unbonded, the HBSN was fitted with a smooth PVC
sheath along the free length only. Once drilling was completed, the grout was flushed from the
annular space between the sheath and the temporary casing. In some cases, the smooth PVC
sheath was slid over the hollow bar after its installation. Flushing of the unbonded length was
performed using a narrow hose and water. Several variations of the flushing tube tips were
attempted, including bending them into a "J" shape and cutting several openings in the side of
the tube as shown in Figure 3. For each variation of flushing tip, water was not permitted to flow
directly toward the grouted bond zone.
Figure 3. Photograph. Outlet end of flushing tube.
After flushing, the casing was withdrawn, allowing the hole to collapse around the PVC and
HBSN in the unbonded length. A schematic diagram of an installed Method B HBSN is shown
in Figure 4. For the test nails installed following this procedure, there is no grout intentionally
surrounding the free length. The depth of flushed grout in the casing was measured using a tape
measure or rod during flushing. However, grout was in the withdrawn casing at depths where
the nail was expected to be fully flushed. This occurrence is noted on the logs. Of particular
11CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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note, significant grout remained in the casing at the Sunset Mesa site where the contractor
elected to wait until the end of the day to pull all the casing for Method B nails as shown in
Figure 5.
Smooth
PVC Sheath
Possible Raveled Soil
After Grout Flushing
Sacrificial Drill Bit
Solid Thread Bar
Structural Grout
Figure 4. Schematic. HBSN installed using Method B.
Figure 5. Photograph. Grout remaining in the
casing after flushing at the Sunset Mesa Site.
12CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Method C: Hollow Bar Installed with Debonding Sheath and Without Flushing
To prohibit grout-to-ground load transfer, it is a common industry practice to place a PVC
bondbreaker onto the HBSN along the length of the bar in the unbonded zone. Upon completion
of the soil nail installation, a body of grout remains in the annular space around the PVC
bondbreaker. For Method C, HBSNs were installed following established installation procedures
as noted in the SOP (FHWA, 2006). Efforts were made to monitor the installation rate and grout
flow for consistency throughout drilling; however, soil conditions and driller preferences had
some impact on the ability to control the consistency of the parameters. Installation rate was
generally consistent at a site but varied from site to site. Grout return was maintained throughout
all drilling.
The structural grout mix was used during drilling and final grouting. Using a single grout
mixture was considered to be a more conservative approach for two reasons: (1) the thinner
grout (typically used during drilling) is more likely to penetrate deeper into granular soils,
resulting in larger grouted zones and thus increased nail capacity, and (2) the single grouting
methods are often utilized by contractors.
Debonding of the hollow bar along the intended free length was accomplished by fitting the bar
with a smooth PVC sheath, which was secured to the coupler using duct tape. Grease was
applied to the surface of the bar before installation of the smooth PVC sheath. A schematic
diagram of an installed Method C HBSN is shown in Figure 6. Only at the Olympia Site, the
smooth PVC sheath was pushed over the bar and through the grout after drilling. Installation of
the smooth PVC sheath after grouting was possible because there was no tendency for collapse
of the drill holes as observed during nail installation using Methods A and B. It is the experience
of the authors that inserting the smooth PVC sheath after installation of the test HBSN may be
difficult or impossible at many sites, and should not be relied upon except in cases where the
required free length is relatively short.
It is noted that for the test nails installed following this method, there is a substantial annulus of
grout that remains around the free length.
13CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Smooth PVC
Sheath
Structural Grout
Sacrificial Drill Bit
Solid Thread Bar
Figure 6. Schematic. HBSN installed using Method C.
Method D: Hollow Bar with Bondbreaker Installed by Re-drilling a Pre-grouted Hole
Method D was performed only at the Olympia Site. The intent of this method was to create a
hole that would be resistant to collapsing without the need for temporary casing, and that could
be subsequently flushed as described in Method B. For Method D, the installer drilled and
grouted the intended free length of the test nail using the same equipment and setup for a typical
HBSN installation. Upon completion of the drilling to the top of the bond zone, the hollow bar
was retracted and the grout allowed to set over night.
This initial grout body was re-drilled within 24 hours during typical HBSN installation
procedures. The intent was to have a soil-cement annulus around the free length of the bar and to
allow flushing of the unbonded zone. At the Olympia Site, redrilling through the day-old grout
was accomplished with little difficulty. The hollow bar did, however, tend to drift toward the
upper left of the initial grout body during drilling, thus resulting in an eccentric annulus with
respect to the pre-drilled hole. However, the hole did remain open and flushing was completed
successfully. A schematic diagram of an installed Method D HBSN is shown in Figure 7.
14CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Smooth PVC
Sheath
Grout Flushed
(No Possible Raveled Soil)
Sacrificial Drill Bit
Solid Thread Bar
Structural Grout
Figure 7. Schematic. HBSN installed using Method D.
CONSTRUCTION DETAILS: MATERIALS, INSTRUMENTATION, AND
INSTALLATION
At each site, except as noted on the logs and summary table, both the SBSNs and HBSNs were
located in the same geologic material. The soil nails were installed at 3 ft to 5 ft center-to-center
spacing along an approximately horizontal line.
Materials
Materials from various suppliers were used. However, they all met certain pre-established
criteria. Bar and drill bits were supplied by several manufacturers including Williams Form,
Con-Tech Systems Ltd. (CTS), and DSI America. Each bar and drill bit type, size, strength, and
manufacturer are noted on the logs. Bar size was determined based on applicability to typical
SBSN and HBSN production methods, as well as on the maximum test load that could be
applied. In general, larger bars were selected to attempt to induce geotechnical failure of the
bond zone of the test nails in order to obtain ultimate bond strength values. Centralizers were
installed on all SBSNs and HBSNs with exception of non-CTS HBSNs. Centralizers are not a
manufactured component of the bar system. Table 2 summarizes materials and dimensions used
at each site for each installation method.
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Table 2. Soil nail material summary.
Casing Casing
Bit
Outer Wall Bar
Site Bit Type Size Bar Type Bar ID
Diameter Thickness Supplier
(in)
(in) (in)
Sacrificial
5 4.5 0.25 Solid # 14 Williams
Wing/Drag
Block 76
Hollow
Carbide 4 4.5 0.25 R51N Williams
Core
Roller 5 7 0.188 Solid # 14 Williams
Posillico
Hollow
Cross Cut 5.1 7 0.188 52/26 mm Con-Tech
Core
Super Jaw 7.75 7 0.43 Solid # 14 Williams
Sunset
Mesa Hollow
Carbide 5 7 0.43 52/26 mm Con-Tech
Core
-- 4 6.25 0.25 Solid # 14 DSI
Olympia
Hollow
Cross Cut 4.52 6.25 0.25 R51N DSI
Core
The grout mix used for this research study was comprised of Type I, Type II, or Type V cement
with a target water-cement ratio of about 0.44 (1 bag of cement per 5 gallons of water). The
grout was mixed using a high shear mixer group pump (typical pump shown in Figure 8). The
specific gravity of the grout was measured before injection into the hole at the end of the tremie
tube, or connection of the hose and swivel. Occasionally the specific gravity of the grout return
was also measured. For nails where a pre-measured volume of grout was placed, no return was
expected or observed. As noted on the drill logs, the specific gravity of the sampled grout was at
least 1.8, as measured using a mud balance.
Grout cubes were also collected and tested for strength. The grout cubes were prepared and
tested in accordance with ASTM C109/C109M-02, Standard Test Method for Compressive
Strength of Hydraulic Cement Mortars (using 2-inch or 50-millimeter cube specimens). As
discussed in a previous section, the same grout mix was used during the entire drilling process,
for both drilling and final grouting.
16CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Figure 8. Photograph. Obermann high shear mixer grout pump.
Strain Gauges
Based on suggestions developed from the review meeting following completion of testing in the
first three sites, strain gauges were utilized only at the Olympia Site. Four Geokon modified
Model 4200 strain gauges were installed into one HBSN for each of the methods used (Methods
B, C, and D) at the Olympia Site. The use of strain gauges was considered in order to measure
axial strain and evaluate the load transfer along the length of the nails.
The strain gauges used were Geokon Model 4200-AX as shown in Figure 9, which have a
relatively small diameter (maximum 19 mm). To reduce the cable diameter, the thermistors were
not activated. The wings of the dumbbell style strain gauge were notched to allow the cables to
pass without having to increase the overall diameter of the strain gauge footprint. On either end
of the dumbbell, Geokon welded a nut, which provided a positive connection for a ¼-inch
diameter all-thread bar. The ¼-inch diameter all-thread bar was used to space and to plunge the
gauges into the grout column within the hollow core bar.
17CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Figure 9. Photograph. Geokon Model 4200-AX strain gauge with
threadbar attachment and notched dumbbells.
Equipment and Installation
The equipment for each site depended on subsurface conditions and equipment availability to the
contractor. The Sunset Mesa, Block 76, and Posillico sites required two different rigs: one to
install the casing as shown in Figure 10, and one to perform the rotary hollow bar drilling. In
general, the equipment used at each site was sufficient to successfully complete the installation.
The equipment used at each site is listed on the respective drill logs. For the installation of the
HBSNs, a pressure gauge was installed at the swivel, as shown in Figure 11. The grout pressure
during installation of the HBSNs is noted on the drill logs.
The installation processes for Methods A, B, C and D were relatively consistent. Several factors
that may affect the bond stress such as rotation rate, advancement rate, grout pressure, grout flow
rate and grout mix data were collected and noted on the drill logs.
18CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Figure 10. Photograph. Casing installation at the
Sunset Mesa Site.
Figure 11. Photograph. Top hammer setup used for
installation Methods B and C. Note the grout pressure gauge
at the swivel and centralizer used to hold PVC in place.
19CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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SOIL NAIL TESTING
The soil nails were tested using typical verification testing methods found in GEC 7 (FHWA,
2003). Typically, verification tests are completed on “sacrificial” nails prior to construction
using the same installation methods as production soil nails.
Verification tests are generally intended to verify a factor of safety of the bond strength, or to
reach ultimate or pull-out loads. For this study, the bonded lengths and bar sizes were designed
to attempt pull-out failure of the test soil nail, based on the presumptive bond strength and drill
hole diameters. Per GEC 7 (FHWA, 2003), “pullout failure is defined as the inability to further
increase the test load while there is continued pullout movement of the test nail.”
For the purpose of establishing load increments, a Design Test Load (DTL) was established for
each test, and is used as a reference in the data collection spreadsheets. The value of DTL was
equal to the intended maximum test load divided by a factor of two. Since the objective was to
reach pull-out failure, the bar was sized to allow the nail to be loaded to more than 150% of the
ultimate load capacity given by the presumptive bond strength. However, in several cases, pull-
out could not be achieved at these high loads.
The load test schedule in Table 3 was used as a starting point for each load test. Soil nail
movement was recorded during each load increment and during creep holds in accordance with
FHWA guidelines. As testing was performed at a given site, the subsequent load test increments
may have been adjusted to collect intermediate data or data beyond the assumed bond strength or
2x DTL. At low loads this hold time was reduced for some tests (see load test logs in
Appendices 1-4). The creep test may have been performed at a different multiple of the DTL,
considering anticipated pull-out load, as shown on the test results.
Table 3. Typical verification test load schedule.
Hold Time
Test Load Increment
(minutes)
AL (0.05DTL max) 1
0.25DTL 10
0.50DTL 10
0.75DTL 10
1.00DTL 10
1.25DTL 10
1.50DTL (creep test) 60
1.75DTL 10
2.00DTL (maximum load) 10
AL 1
Notes: AL = Alignment load
DTL = Design test load
20CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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Load Testing Setup
The load testing setup varied from site to site but consisted of the following critical components:
two dial gauges, dial gauge support, jack and pressure gauge, and a reaction frame. Pressure
gauges were graduated in increments not greater than 100 psi (689.5-kPa). The dial gauges were
capable of measuring to 0.001 inches (0.025 millimeters). The Sunset Mesa typical load test
setup is shown in Figure 12.
Figure 12. Photograph. Load test setup at Sunset Mesa Site.
The reaction for the test load was generally provided by a steel plate supported on wood
cribbing, which was laid out to provide a flat surface perpendicular to the nail alignment.
Cribbing and plates spread the load over an area of at least nine square feet. Where shallow
benches were constructed for nail installation, additional efforts were required to prevent passive
failure of the wall face during jacking.
Strain Gauge Use at Olympia Site
Four Geokon 4200-AX strain gauges were installed into the HBSNs in order to measure axial
strain and evaluate the load transfer along the nails. The gauges were installed inside the hollow
portion of the bar after final grouting. The hollow core provided confinement of the grout in
which the strain gauges were installed. One gauge was located within the unbonded length,
while the remaining four gauges were distributed within the bond zone. The gauge inside the
unbonded length was intended to detect load shedding that might occur along the unbonded
length.
21CHAPTER 3 – FIELD STUDY AND ANALYSIS PROGRAM
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The load at each strain gauge was calculated using Equation (1):
P = εaAE (1)
where:
P = Force in nail
εa = Axial strain as measured by the gauge
E = Young’s Modulus of steel, 29,000 ksi
A = Cross-sectional area of the steel reinforcing bar
During initial loading of the soil nail, before the grout develops generalized tensile cracking, this
equation underestimates the axial load of the soil nail. However, the interpretation of the strain
gauge data focused in determining the bond strength along the soil nail under relatively large test
loads, and once the contribution of the grout to the axial stiffness has become negligible or zero.
22CHAPTER 4 – FIELD TEST RESULTS
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CHAPTER 4 – FIELD TEST RESULTS
OVERVIEW OF DATA
The appended Data Reports provide details about each test site, nail layout, soil conditions, and
test results; and the reader is directed to the appendices for further details. A summary of the
load test data is shown in Tables 4 through 7 for the four sites to provide a ready comparison of
the information for the purposes of this summary report. The following notes explain the column
headings for Tables 4 through 7.
Column Descriptions:
Nail Number/Installation Method: Nail reference number from the field and installation
method as discussed in Chapter 3.
Maximum Test Load: Jack load at soil nail pullout if pullout occurred; otherwise
maximum test load.
Maximum Movement: Cumulative movement measured at the soil nail tail with dial
gauges at the maximum test load just prior to unloading.
Residual Movement: Cumulative movement measured at the soil nail tail with dial
gauges after unloading to alignment load at maximum test load.
Pullout Failure Achieved?: "Yes" indicates pullout failure achieved at the maximum test
load. "No" indicates pullout failure was not achieved at the maximum test load.
Bond Length: The distance from the tip of the bar farthest from the soil face to the edge
of the coupler or smooth-walled PVC, whichever is shorter.
Average Drilling Rate (ft/min): Average of the drilling rate measurements within the
bond length.
Calculated Average Bond Strength (psi): Bond strength as calculated per Equation (2),
which follows Table 7.
Soil Type Along Bond Zone: Soil type encountered in the bond zone based on nearby
borings and drill cuttings observed during soil nail drilling. Soil density / consistency is
based on nearby borings. The soil type is designated as Group Symbols in accordance
with ASTM D2487 Unified Soil Classification System.
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